1. Field of the Invention
This invention relates to a self-plugging blind fastener, such as a blind rivet or blind bolt, of the kind comprising a headed tubular body and an elongate stem which, in order to set the fastener, is pulled along the bore of the body so as to deform the body to form a blind head and wherein at least part of the stem is positively locked in the bore of the body of the set fastener so that the part is retained as a plug, closing the bore and contributing strength to the fastener, without risk of the plug becoming separated from the body.
2. Discussion of the Background
Examples of such fasteners are described in British Patents Nos. 1 538 872 and 2 127 514.
In GB 1 538 872, the stem has a plug part provided with an annular skirt surrounding an inner core of the plug part, and, during setting of the fastener, the free end of the skirt is deformed so as to flare radially outwardly and to engage an abutment surface of the body which faces axially towards the head end of the body. Thus the plug part is locked in the bore of the body by abutment of the flared end of the skirt and the abutment surface of the body.
In GB 2 127 514, the fastener includes a stem, or "pin", having an annular lock pocket and an annular stop shoulder adjacent to the lock pocket, and a headed tubular body or "sleeve" having a bore which is reduced in diameter near the head of the sleeve to provide an annular stop shoulder, the stop shoulder being separated from the surrounding or "radially confronting" portion of the sleeve by an annular slit which extends axially towards the head of the sleeve, and terminating at a radially inwardly, rearwardly inclined, generally frusto-conical surface. During setting of the fastener, the annular stop shoulder of the pin engages the frusto-conical surface of the stop shoulder of the sleeve, and causes the sleeve stop shoulder to be folded or moved so that its free end moves radially inwardly into the lock pocket of the pin, and thus serves to lock the plug part of the pin in the sleeve.
In both the exemplary fasteners of the prior art referred to above, the locking of the plug part in the body of the fastener is fairly satisfactory, but can be overcome simply by reversing the movement of the material which was initially deformed to provide the locking engagement.
Thus, a moderate force tending to push the plug out of the body, such as might be encountered in rigorous conditions of use, can be sufficient to produce such a reverse movement of the locking material so that it is swept out of locking engagement with the surface it is meant to engage.
This is probably due to the fact that the locking material is provided by a free end portion of a hollow cylinder, such as the annular skirt or sleeve stop shoulder, which, to form the lock, is flared radially either outwardly or inwardly, and it is a relatively easy matter then to deform it back to its original shape.
It is desirable that the locking together of the plug and body be so strong that these parts can only be separated by shearing, rather than bending, material forming the lock. It is also desirable that the force required to form such a lock should not be very great, since it is necessary for all of the setting force to be generated by the tool used for setting the fastener, and it may also be necessary for it to be transmitted-by a breakneck of limited strength which is required to break when the setting of the fastener is complete.